Found this on ELite--it is Big Cats profile on Proviron, Amended by Lawnsaver.

Mesterolone is an orally active, 1-methylated DHT. Like Masteron, but then actually delivered in an oral fashion. DHT is the conversion product of testosterone at the 5-alpha-reductase enzyme, the result being a hormone that is 3 to 4 times as androgenic and is structurally incapable of forming estrogen. One would imagine then that mesterolone would be a perfect drug to enhance strength and add small but completely lean gains to the frame. Unfortunately there is a control mechanism for DHT in the human body. When levels get too high, the 3alpha hydroxysteroid dehydrogenase enzyme converts it to a mostly inactive compound known as 3-alpha (5-alpha-androstan-3alpha,17beta-diol), a prohormone if you will. It can equally convert back to DHT by way of the same enzyme when low levels of DHT are detected. But it means that unless one uses ridiculously high amounts, most of what is administered is quite useless at the height of the androgen receptor in muscle tissue and thus mesterolone is not particularly suited, if at all, to promote muscle hypertrophy.

Proviron has four distinct uses in the world of bodybuilding. The first being the result of its structure. It is 5-alpha reduced and not capable of forming estrogen, yet it nonetheless has a much higher affinity for the aromatase enzyme (which converts testosterone to estrogen) than testosterone does. That means in administering it with testosterone or another aromatizable compound, it prevents estrogen build-up because it binds to the aromatase enzyme very strongly, thereby preventing these steroids from interacting with it and forming estrogen. So Mesterolone use has the extreme benefit of reducing estrogenic side-effects and water retention noted with other steroids, and as such still help to provide mostly lean gains. Its also been suggested that it may actually downgrade the actual estrogen receptor making it doubly effective at reducing circulating estrogen levels.

The second use is in enhancing the potency of testosterone. Testosterone in the body at normal physiological levels is mostly inactive. As much as 97 or 98 percent of testosterone in that amount is bound to sex hormone binding globulin (SHBG) and albumin, two proteins. In such a form testosterone is mostly inactive. But as with the aromatase enzyme, DHT has a higher affinity for these proteins than testosterone does, so when administered simultaneously the mesterolone will attach to the SHBG and albumin, leaving larger amounts of free testosterone to mediate anabolic activities such as protein synthesis. Another way in which it helps to increase gains. Its also another part of the equation that makes it ineffective on its own, as binding to these proteins too, would render it a non-issue at the androgen receptor.

Thirdly, mesterolone is added in pre-contest phases to increase a distinct hardness and muscle density. Probably due to its reduction in circulating estrogen, perhaps due to the downregulating of the estrogen receptor in muscle tissue, it decreases the total water build-up of the body giving its user a much leaner look, and a visual effect of possessing "harder" muscles with more cuts and striations. Proviron is often used as a last-minute secret by a lot of bodybuilders and both actors and models have used it time and again to deliver top shape day in day out, when needed. Like the other methylated DHT compound, drostanolone, mesterolone is particularly potent in achieving this feat.

Lastly Proviron is used during a cycle of certain hormones such as nandrolone, with a distinct lack of androgenic nature, or perhaps 5-alpha reduced hormones that don't have the same affinities as DHT does. Such compounds, thinking of trenbolone, nandrolone and such in particular, have been known to decrease libido. Limiting the athlete to perform sexually being the logical result. DHT plays a key role in this process and is therefore administered in conjunction with such steroids to ease or relieve this annoying side-effect. Proviron is also commonly prescribed by doctors to people with low levels of testosterone, or patients with chronic impotence. Its not perceived as a powerful anabolic, but it gets the job done equally well if not better than other anabolic steroids making it a favorite in medical practices due to its lower chance of abuse.

Mesterolone is generally well liked nonetheless as it delivers very few side-effects in men. In high doses it can cause some virilization symptoms in women. But because of the high level of deactivation and pre-destination in the system (albumin, SHBG, 3bHSD, aromatase) quite a lot of it, if not all simply never reaches the androgen receptor where it would cause anabolic effects, but also side-effects. So its relatively safe. Doses between 25 and 250 mg per day are used with no adverse effects. 50 mg per day is usually sufficient to be effective in each of the four cases we mentioned up above, so going higher really isn't necessary. Unlike what some suggest or believe,

I will post an abstract to refute these next statements at the bottom of the page

Its not advised that Proviron be used when not used in conjunction with another steroid, as it too is quite suppressive of natural testosterone, leading to all sorts of future complications upon discontinuation. Ranging from loss of libido or erectile dysfunction all the way up to infertility. One would not be aware of such dangers because Proviron fulfills most of the functions of normal levels of testosterone.

Stacking and Use:

Mesterolone is an oral alkylated steroid. If used primarily as an anti-aromatase drug, using it throughout a longer cycle (10-12 weeks) of injectables may elevate liver values a little bit, though much, much less than one would expect with a 17-alpha-alkylated steroid. Eventhough instead of inhibiting gains, mesterolone may actually contribute to gains. So that's a bit of a shame. Its not quite as toxic since its not alkylated in the same fashion, but at the 1 position, which reduces hepatic breakdown, but not like 17-alpha alkylation. The reason for the change of position I assume, is because alkylating at the 17-alpha position has been shown to reduce affinity for sex hormone binding proteins. This would in turn decrease its ability to free testosterone. Nonetheless the delivery rate is quite good. Its taken daily in 50-100 mg doses.

The best thing to stack it with is testosterone of course. Its most easily bound to SHBG and albumin, and deactivated for up to 98%. Since the DHT can compete for these structures with higher affinity it would naturally lead to a higher yield of whatever testosterone product you stacked it with. Since DHT levels are notably higher now there is also more stimulation of the androgen receptor causing more strength gains, and because of its affinity for aromatase the overall estrogen level decreases as well. This has as a result that gains are leaner, and once again the overall testosterone yield is increased as less I converted at the aromatase enzyme.

It's of course used in other stacks with products such as methandrostenolone, boldenone and nandrolone to reduce estrogenic activity and increase muscle hardness. The addition of proviron makes boldenone a dead lock for a cutting stack and for some may even make it possible to use nandrolone while cutting, although the use of Winstrol or a receptor antagonist in conjunction is wishful as well. The benefit of adding it to a nandrolone stack is that it may also help you reduce the decrease in libido suffered from nandrolone, since the latter is mostly deactivated by 5-alpha reductase, an enzyme that makes other hormones more androgenic.

Proviron is an anti-aromatase, so obviously anti-estrogens would be futile and redundant. Blood pressure medication for those prone to hypertension may be wise, as this DHT can increase the blood pressure.

Abstract refuting that Proviron is not highly suppressive

Here is the study I was referring to. Only 85 men out of 250 showed any suppression. Proviron did not shut down the HPTA in any of the subjects and that was at 150mg for 1 year. I would say its pretty safe and has very little effect on one's HPTA

This study shows no effect on normal LH and FSH with 100-150mg/ d mesterolone, and decrease of FSH/LH that were elevated.
Proviron doesn't substitute Clomid as hpta therapy, but doesn't get in the way, either.
The effect of mesterolone on sperm count, on serum follicle stimulating hormone, luteinizing hormone, plasma testosterone and outcome in idiopathic oligospermic men.

Two hundred fifty subfertile men with idiopathic oligospermia (count less than 20 million/ml) were treated with mesterolone (100-150 mg/day) for 12 months. Seminal analysis were assayed 3 times and serum follicle stimulating hormone (FSH) luteinizing hormone (LH) and plasma testosterone were assayed once before treatment and repeated at 3, 6, 9 and 12 months after the initiation of treatment. One hundred ten patients (44%) had normal serum FSH, LH and plasma testosterone, 85 patients (34%) had low serum FSH, LH and low plasma testosterone. One hundred seventy-five patients (70%) had moderate oligospermia (count 5 to less than 20 million/ml) and 75 patients (30%) had severe oligospermia (count less than 5 million/ml). Seventy-five moderately oligospermic patients showed significant improvement in the sperm density, total sperm count and motility following mesterolone therapy whereas only 12% showed improvement in the severe oligospermic group. Mesterolone had no depressing effect on low or normal serum FSH and LH levels but had depressing effect on 25% if the levels were elevated. There was no significant adverse effect on testosterone levels or on liver function. One hundred fifteen (46%) pregnancies resulted following the treatment, 9 of 115 (7.8%) aborted and 2 (1.7%) had ectopic pregnancy. Mesterolone was found to be more useful in patients with a sperm count ranging between 5 and 20 million/ml. Those with severe oligospermia (count less than 5 million) do not seem to benefit from this therapy.

PMID: 2892728 [PubMed - indexed for MEDLINE]

One more...
Effect of non aromatizable androgens on LHRH and TRH responses in primary testicular failure.

Spitz IM, Margalioth EJ, Yeger Y, Livshin Y, Zylber-Haran E, Shilo S.

We have assessed the gonadotropin, TSH and PRL responses to the non aromatizable androgens, mesterolone and fluoxymestrone, in 27 patients with primary testicular failure. All patients were given a bolus of LHRH (100 micrograms) and TRH (200 micrograms) at zero time. Nine subjects received a further bolus of TRH at 30 mins. The latter were then given mesterolone 150 mg daily for 6 weeks. The remaining subjects received fluoxymesterone 5 mg daily for 4 weeks and 10 mg daily for 2 weeks. On the last day of the androgen administration, the subjects were re-challenged with LHRH and TRH according to the identical protocol. When compared to controls, the patients had normal circulating levels of testosterone, estradiol, PRL and thyroid hormones. However, basal LH, FSH and TSH levels, as well as gonadotropin responses to LHRH and TSH and PRL responses to TRH, were increased.

There was, however, a reduction in the integrated and incremental TSH secretion after TRH.
Fluoxymesterone administration was accompanied by a reduction in thyroid binding globulin (with associated decreases in T3 and increases in T3 resin uptake). The free T4 index was unaltered, which implies that thyroid function was unchanged.

In addition, during fluoxymesterone administration, there was a reduction in testosterone, gonadotropins and LH response to LHRH.

Basal TSH did not vary, but there was a reduction in the peak and integrated TSH response to TRH. PRL levels were unaltered during fluoxymesterone treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Proviron is a very good product to use it is cheap and works great. Enhances gains but may NOT always be enough to prevent gyno. So if you are particularly gyno prone nolva may still be needed.
Proviron for many causes raises in blood pressure so keep an eye on it. Personally, it does not effect mine at 50mg ed.